Fuel systems for aircraft gas turbine engines

ABSTRACT

A fuel system for a gas-turbine engine including a source of fuel at pressure, a flyweight device driven at a speed proportional to engine speed, a first-adjustable fuel flow means adjusted by centrifugal movement of the flyweight device, manually adjustable spring means acting on the flyweight device to oppose centrifugal movement at least over a substantial upper range of engine speeds, a second-adjustable fuel flow means, a single-manually operable means jointly controlling the spring means and the second fuel flow means such that at low altitudes for the lower range of engine speeds, the centrifugal force developed by the flyweight device is unable to overcome the load of the spring whereby the fuel flow to the engine from said source is determined by the second fuel flow means and such that for the higher range of engine speeds centrifugal force developed by the flyweight device is able to overcome the loading on the adjustable spring means whereby the first fuel flow means is operable to determine fuel flow from said source to the engine.

United States Patent [191 Robinson FUEL SYSTEMS FOR AIRCRAFT GAS TURBINEENGINES [75] Inventor: Keith Robinson, Churchdown,

England [73] Assignee: Dowty Fuel Systems Limited,

Cheltenham, England [22] Filed: Feb. 29, 1972 [21] Appl. No.: 230,499

Cushman May 7,1974

[57] ABSTRACT 'A fuel system for a gas-turbine engine including a means,a single-manually operable means jointly con-- trolling the spring meansand the second fuel flow means such that at low altitudes for the lowerrange of engine speeds, the centrifugal force developed by the flyweightdevice is unable to overcome the load of the spring whereby the fuelflow to the engine from said source is determined by the second fuelflow means and such that for the higher range of engine speedscentrifugal force developed by the flyweight device is able to overcomethe loading on the adjustable spring means whereby the first fuel flowmeans is operable to determine fuel flow from said source to the engine.

11 Claims, 6 Drawing Figures- PATENTEUIAY 7 I974 SHEET 1 OF 4 0 FLUW RATE FUEL SYSTEMS FOR AIRCRAFT GAS TURBINE ENGINES This invention relatesto a fuel system for an aircraft gas-turbine engine and particularlyalthough not exclusively for a two-spool gas-turbine engine. A two-spoolgas-turbine engine is one in which there are two separate spools orrotors mounted co-axially one relatively to the other, the low pressurespool including a low pressure compressor and a low pressure turbine andthe high pressure spool including a high pressure compressor and a highpressure turbine, the high pressure spool being mounted intermediate thelow pressure compressor and the low pressure turbine so that air may becompressed in series by the low pressure compressor and the highpressure compressor and fed to a combustion chamber, the gas from thecombustion chamber feeding in series to the high pressure compressor andthe low pressure compressor.

With a gas-turbine engine and particularly a twospool gas-turbine engineit is important that the top speed should not be exceeded and it is alsoimportant to control the speed accurately in the upper part of the speedrange within which the engine normally operates. For this purpose it isconventional to provide a fly weight kind of top speed governor and aflyweight kind of range speed governor normally adjustable to selectspeeds in the upper part of the engine speed range.

The present invention sets out to simplify this arrangement by providingonly one flyweight governor device to provide both top and range speedgoverning function s.

In accordance-with the present invention a fuel system for a gas-turbineengine includes a fuel flow source connected to deliver fuel to theengine, a flyweight device driven at a speed proportional to enginespeed, a first adjustable fuel flow means arranged by centrifugalmovement of the flyweight device to vary said fuel flow to the engine;manually adjustable spring means acting on the flyweight device tooppose centrifugal movement at least over a substantial upper range ofengine speeds, a second adjustable fuel flow means to vary said fuelflow to the engine, a single. manually operable means jointlycontrolling the spring means and the second fuel flow means relatingrelative adjustment of the spring means and the second adjustable fuelflow means during adjustment of said single manually operable means, anda third adjustable fuel flow means r'esponsive to instantaneous valuesof at least one engine function to limit fuel flow to the engine duringengine acceleration following movement of said manually operable meansto increase fuel flow to the engine.

The flyweight device and spring means may be so constructed to provideaccurate speed governing over a small upper speed range of the enginegiving accurate speed control of the engine where it is principallyrequired and also giving an accurate top speed control.

. Where the engine is a two-spool engine the flyweight device ispreferably driven by the low pressure spool.

The fuel system may include a first variable throttle through which fuelflows to the engine and a first bypass valve arranged to by-pass fuelfrom the throttle to maintain a controlled pressure drop across thethrottle, and the first adjustable fuel flow means may then comprise afirst adjustment means connecting the flyweight device to adjust thefirst throttle and the third. adjustable fuel flow means-may thencomprise an engine function responsive means and a second adjustmentmeans connecting the function responsive means to ad'- just the firstthrottle, and discriminator means to select one of the first and secondadjustment means to control said throttle.

The engine function responsive means may respond to at least one enginegas pressure.

The first by-pass valve may include means responsive to engine speedarranged to adjust by-pass fuel flow from the first throttle so that thepressure drop at the first throttle is in accordance with the square ofengine speed.

One embodiment of the invention will now be described with reference tothe accompanying drawings, in which FIG. 1 is a diagrammatic elevationof an aircraft including a two-spool gas-turbine engine and stabilisingjets,

' FIG, 2 is a diagrammatic plan view of the engine in FIG. 1 and thegallery system feeding the burners which forms part of the presentinvention,

FIG. 3 is a graph showing a desired relation between fuel pressure andflow rate obtained by the arrangeparts joinable together, the partsbeing indicated as FIGS. 4A, 4B and 4C. I

Reference is made initially to FIG. 1 of the drawings. The aircraft isconventionally provided with a fuselage l, wings 2 and 3, tail planes 4and a rudder. The engine 5 mounted in the aircraft is of thetwospool.kind embodying a low pressure compressor A, a high pressurecompressor B, combustion chambers C, a high pressure turbine D and a lowpressure turbine E. The compressor A, and turbine E are mounted on asingle shaft for rotation together whilst the compressor B and turbine Dare mounted on another shaft for rotation together co-axially withcompressor A and turbine E. The low pressure compressor deliverscompressed air both to the high pressure compressor B and to a pair ofswivelling nozzles 6 which may be adjusted to produce either horizontalor vertical thrust. The air delivered by the high pressure compressor Benters the combustion chambers C and the high temperature gas leavingthe combustion chambers passes the high pressure turbine and the lowpressure turbine in succession. The exhaust gases from the low pressureturbine are fed to a second pair of swivelling nozzles 7 adjustable withthe nozzles 6 to provide vertical or horizontal thrust.

A bleed connection 8 from the high pressure compressor B connects to aduct system comprising'four ducts 9, ll, 12 and 13 leading respectivelyto stabilising trol surfaces on the wings, tail and rudder. The nose jet16 is provided with an obturator also controlled b y the pilots controlleading to the tail control surfaces. An aircraft of this nature isalready known and one example may be seen in US. Pat. No. 3,160,368.

In order to feed fuel to the combustion chambers, a fuel galleryarrangement is provided as diagrammatically illustrated in FIG. 2. Fuelenters the engine at a pipe 19 and passes to a pair of pressure reliefvalves 21 and 22, the valve 21 having a lower effective spring loadingthan the valve 22. The deliveries from valves 21 and 22 connectrespectively to a pair of circular galleries 23 and 24 encircling theengine at the location of the combustion chambers. A number ofvapourising burner assemblies 25 are provided around the engine feedingto the combustion chambers. Each burner assembly includes a vapourisingtube 26 fed with fuel from a swirl chamber 27. A connection 28 fromgallery 23 enters the swirl chamber tangentially and a second pipe 29from the gallery 24 enters the swirl chamber axially. The main purposeof the swirl chambers is to equalise flow to all burner assemblieswithout involving small diameter orifices which could become choked withsolid contaminants in the fuel. Fuel from the pipe 28 produces the basicswirl in the chamber 27 and fuel which enters from the pipe 29 is alsothereby swirledby virtue of the swirling fuel already within the chamber27. The lower pressure loading of the valve 21 ensures that for low flowrates fuel enters only through valve 21, gallery 23, pipes 28 and theswirl chambers 27. When the fuel pressure reaches a predetermined'valuethe valve 22 will open carrying further fuel into the swirl chambersthrough galleries 24 and pipes 29. The main function of the valves 21and 22 is to provide an almost linear relation between fuel pressure inpipe 19 and the actual total flow rate of fuel into the engine. A simpleswirl chamber device on its own would produce a square-lawcharacteristic which over the range of fuel flows required for theengine would produce an unacceptably high pressure at maximum fuel flow.Additionallythe near linear relation between pressure and fuel flow rateis usefully employed within the fuel control system of FIG. 4 as will bedescribed in order to assist in the stable governing of the engine.

Reference is now made to FIG. 4 of the accompanying drawings. Fuel isdelivered from the aircraft fuel tank in a conventional manner by meansof a boost pump (not shown) and an engine-driven gear pump 31. Fuelenters the pump at pipe 32 and leaves at pipe 33. A pressure reliefvalve 34 by-passes fuel above a predetermined maximum pressure from thedelivery pipe 33 through a pipe 35 and back into a boost pressure pipenetwork 36 which connects back to the pump inlet 32. The fuel from pipe33 initially enters an emergency change-over valve 37 which is capableof delivering the flow from pipe 33 into either the normal delivery pipe38 or the emergency delivery pipe 39. The controlling spool 41 withinvalve 37 is servo-adjusted by means of a piston 42 slidable within acylinder 43 and a vent valve 44 adjusted by a solenoid 45. High pressureliquid from pipe 33 has access within the spool 41 to a central passage46 from which fuel may flow both to the lower end of the spool 41 andthrough a restrictor 47 to the cylinder 43. When valve 44 opens to ventcylinder 43 fuel at pressure acting on the lower end of the spool 41will push the spool 41 upwardly to connect the delivery 33 to theemergency pipe 39. Normally the valve 44 remains closed enablingpressure to exist in cylinder 43 to hold the spool 41 in its lowermostposition in which the delivery pipe 33 isconnected to the pipe 38.

The pipe 38 connects to a filter 48 whose function is to deliver to pipe49 a small filtered flow of fuel suitable for servo use within the fuelsystem. The main delivery from pipe 38 however leaves filter 48 at pipe51 to enter the first throttle valve 52.

Within valve 52 the throttle element comprises a hollow piston 53slidable in a valve cylinder to control the opening between a port 54connected to the inlet pipe 51 and a port 55 connected to an output pipe56. Within the hollow piston, ports 57 permanently connect the hollowpiston to the port 54 whilst a throttle aperture 58 overlaps the port 55to form a variable throttle whose throttling effect is determined by theposition adopted by the piston 53 relative to the port 55. In order toadjust the piston 53 a servo piston 59 is secured to it for slidingmovement within a cylinder 61.

The servo supply pipe 49 connects directly to the upper end 62 of thecylinder 61 and indirectly through a restrictor 63 to the lower end 64of cylinder 61. It will be seen that the upper effective area of piston59 exposed to full pressure in the end 62 of the cylinder is smallerthan the lower area of piston 59 exposed to the end 64 of the cylinder.

The adjustment of the hollow piston 53 is by virtue of two separate ventcontrols acting on vent pipes 65 and 66. In order to ensure that theservo piston 59 can never be controlled by the joint effect ofsimultaneous venting of the pipes 65 and 66, a discriminator 67 isprovided. The discriminator comprises upper and lower chambers 68 and 69separated by a flexible diaphragm 71. The diaphragm 71 is normallyspringloaded against a vent 72 in chamber 69 such that if the diaphragm71 lifts from vent 72 the lower chamber 69 is connected to the lowpressure pipe 36 via passage73. The vent pipe 65 and the lower end 64 ofcylinder 61 are directly connected to the chamber 69. The upper chamber68 is directly connected to vent passage 66 and also through arestrictor 74 to the upper end 62 of cylinder 61.

If the vent pipe 65 is vented, the pressure in the lower chamber 69 isreduced ensuring that pipe 65 is directly connected to the lower end 64of cylinder 61 and that also that diaphragm 71 closes vent 72. On theother hand if the vent pipe 65 is closed but pipe 66 is vented, thediaphragm 71 will lift to open the vent 72 a small amount which willallow a controlled reduced pressure in accordance with the reducedpressure in vent 66 to be generated in lower chamber 69 and directly fedto the lower end 64 of cylinder 61. Thus either of the vents 65 or 66may generate a controlling pressure at the lower end 64 of cylinder 61.

In order to control the vent pipe 66 the upper part of the throttlevalve 52 is provided with a chamber 75 within whicha lever 76 ispivotally mounted at a fulcrum 77. The lever 76 carries a conventionalhalf-ball valve 78 controlling flow from the vent pipe 66 into thechamber 75 which is in turn connected to the low pressure network 36.Within chamber 75 a bellows assembly is provided comprising threebellows 79, 81 and 82 and an inter-connecting yoke 83 which bearsthrough pin 84 onto the lever 76. The bellows 79 is connected to a pipe85 and receives a signal derived from the high pressure (P3) deliveredby the high pressure compressor B. The bellows 81 is connected to a pipe86 and receives a signal of the delivery pressure of the low pressurecompressor A (P2). The bellows 82 is evacuated. The bellows are mountedby being secured within the chamber 75 to the yoke 83 so that the forceexerted on the lever is a function of the difference of the pressure P2and the pressure derived from P3 and forms part of an acceleration fuelflow controlling system.

Also within the chamber 75 a feed-back spring 88 is provided, beingcompressible by movement of the throttle piston 53 so as to exert on thelever 76 a force dependent on the position of piston 53. An adjuster 89is provided for pre-set datum adjustment of the loading of the spring byadjusting the load in a spring 87 acting on lever 76 in opposition tospring 88.

The thrust exerted by the pin 84 on lever 76 determines the escape flowpermitted from the vent 66 into chamber 75. Variation of the pressuresignal on the pin 84 will alter the position of the lever 76 which willin turn alter the pressure at the lower end 64 of cylinder 61 to movepiston 59 and piston 53 in the sense to alter the compression of spring88 so that the changed force exerted by the spring will compensate forthe change of force exerted by the pin 84 thereby restoring thehalf-ball valve 78 to a predetermined clearance relative to the vent 66.in other words the position of the piston 53 will be in proportion tothe force exerted by the pin 84 on the lever 76. The describedpositional control for the throttle piston 53 by virtue of the lever 76will only be effective provided that the vent 65 is closed. Thedescription of the control for vent 65 will appear further in thisspecification.

The control of fuel flow by the throttle piston 53 is made fullyeffective by providing a by-pass valve 91 which operates to control aby-pass flow of fuel from the pipe 38 through a by-pass pipe 92 and alow pressure relief valve 93 into the low pressure network 36. Theby-pass valve is basically formed bya hollow piston 94 which controlsthe flow between a pair of ports 95 and 96 in a co-operating cylinder. Aport 97 in the hollow piston connects permanently to the port 95 duringpiston movement and a throttle aperture 98 in overlapping relation withthe port 96 determines an adjustable throttle controlling the flow offuel through the by-pass pipe 92 and valve 93 into the low pressurenetwork 36. Axial adjusting movement of the piston 94 is provided by twoopposed signals. The first of these signals is a pressure drop signalbetween two differing pressures existing in a chamber 99 enclosing theupper end of piston 94 and in a chamber 101 enclosing the lower end ofpiston 94. The chamber 101 is connected by pipe 102 to the port 55 inthe throttle valve 52 and carries the pressure downstream of thethrottle valve 52. The upper chamber 99 receives pressure from a pipe103 connected to the pipe 51 upstream of the throttle valve 52.

For the purpose of adjustment in setting the system up for operation,means are provided for adjusting the relationship between the pressuredrop at throttle 52 and the downward force acting on the hollow piston94, these means comprising a variable restrictor 104 and a 6 ings and isadapted to be driven by the high pressure spool of the engine. The shaft106 enters the chamber fixed restrictor 105 connected in series betweenthe pipes 102 and 103, the space 99 being connected to the junctionbetween the restrictors 104 and 105. In effect In the lower part of thehousing for the by-pass valve 91 a rotary drive shaft 106 is suitablymounted in bear- I 101 and carries a pair of gears 10 7 and 108. Thegear 108 is of substantial axial length and meshes with a gear 109 torotate piston 94 to reduce its friction to axial movement. The gear 109has a larger number of teeth than the gear 108 so that piston 94 rotatesat a comparatively slow speed. The gear 107 meshes with a gear 1 1 1 torotate a carrier 1 12 in a lower chamber 1 10. On the carrier 112 a setof flyweights 113 are pivotally mounted for centrifuging movement. Theflyweights are so constructed that their overall density is about twicethe mean density of the range of fuels to be used. The flyweights may bemade of metal with internal completely closed cavities to provide thecorrect overall density. The lower chamber 110 is connected to chamber111 and by virtue of its pipe connections receives a flow of fuel whichat any instant can be said to have the same density as the fuel flowingto the engine. The flyweights will cause rotation of fuel in chamber 110and the centrifugal force generated on the flyweights will beproportional to the difference in overall density of the flyweights anddensity of the fuel such that with reducing fuel density the centrifugalforce becomes higher and vice versa.

The flyweights 113 are provided with inwardly directed levers 114 toexert an axial thrust on a central cup 115. The cup 115 contains aspring 116 which reacts against a push rod 117 extending from the piston94. The spring 116 and the cup 115 are so arranged that at low enginespeeds the centrifugal force generated by the flyweights 113 cannotovercome the load of spring 116 and the flyweights are held againstinward stops on the carrier by the spring load. The spring force thenacts as a minimum force on piston 94. At higher speedswhen thecentrifugal force is higher than the load of spring 116 the cup 115makes direct contact with the rod 117 so that centrifugal thrust istransmitted directly to the piston 94 in opposition to the pressure dropforce exerted on piston 94 by virtue of the pressures in chambers 99 and101. It will be appreciated that the flyweights 113 must be capable of asubstantial range of radial movement in order to effect control movementof the hollow piston 94. The ends of the control levers 114 which reacton the hollow cup 1 15 are so shaped that as the flyweights 1 13 moveoutwardly the mechanical leverage of levers 114 is reduced thus toensure that the thrust exerted by the flyweights 113 on the hollowpiston94 is always in substantially constant proportion to the square of therotational speed of the high pressure spool in the engine for any fixedvalue of fuel density at the flyweights.

The operation of the by-pass valve 91 is to ensure that the pressuredrop occurring at the throttle valve 52 is in proportion to thecentrifugal force exerted by the flyweights 113. The square-lawcharacteristic between pressure drop and flow at the throttle aperture58 will then ensure that the mass flow rate of fuel passing through thethrottle aperture 58 is directly proportional to the speed of the highpressure spool for any one setting of the throttle 58 independently ofnormal fuel density changes, the surplus fuel being by-passed from pipe38 through pipe 92 and the adjustable throttle aperture 98 back to thelow pressure network 36. The function of the spring 116 at low enginespeeds is to ensure a small excess of fuel at light-up by providing aminimum force acting on piston 94 in opposition to the pressure dropforce. Over the range of normal running speeds, the cup 115 willdirectly contact the rod 117 ensuring that the centrifugal force fromflyweights 113 is directly transmitted to piston 94.

The acceleration fuel flow determined by the throttle 52 is in partdetermined by the pressure drop function applied by the by-pass valve 91and in part by the setting of the piston 53 by virtue of the bellowsassembly 79, 81, 82. The throttle aperture 58 may have a suitableshaping to ensure that acceleration fuel flow under all conditions ofoperation is just below the value which could cause compressor stall.

Acceleration fuel metering by the throttle 52 takes place only underconditions when the vent 65 is closed. The vent 65 extends to themechanical governor unit 121. Within the governor unit 121 a chamber 122is provided within which a lever 123 is pivotally mounted at a fulcrum120. A number of forces are arranged to act on the lever 123, theprincipal being the centrifugal force given by a set of flyweights 124which are rotatably driven by ashaft 125 from the low pressure spool inthe engine. The flyweights 124 are of a well-known kind which provideinsensitivity of output force to any variation in density of liquidwithin which they rotate. As is usual with aircraft engine fuel controlsystems the flyweights are very conveniently submerged in fuel so thatinsentivity to variation in fuel density is obtained. The centrifugalforce due to fiyweightrotation is transmitted through a rod 126 to acton the lever 123. An adjustable compression spring 127 acts in theopposite direction on lever 123, the loading of this spring beingadjusted by a cam 128 which is mechanically connected for adjustment bythe pilots control lever 129. The lever 123 acts on a half-ball valve131 which controls the escape flow from the vent pipe 65. it will beseen that fuel escaping from the vent pipe 65 enters the chamber 122from which it will have access to the flyweights 124 and also will beable to escape to the low pressure pipe network 36. The lever 123 andhalf-ball valve 131 are so arranged that a very small movement(approximately 0.002 inches) is necessary at the halfball valve toadjust the vent pipe 65 between the fully closed and the fully openposition. This ensures that the range of centrifugal movement of thefiyweights 124 is kept to an absolute mimimum to assist in accuratespeed governing. When the half-ball valve 13] opens slightly to providegoverning control, the vented fuel from the lower chamber 69 in thethrottle valve 52 will reduce the pressure in chamber 69 and the lowerend 64 of cylinder 61 thus causing diaphragm 71 to close vent 72. Whenvent 72 is closed the control of movement of the piston 53 is entirelyunder the control of the half-ball valve 131. The fact that the pressuredrop across the throttle 52 is controlled to the square of high pressurespool speed is then of no consequence since the pressure drop is onlyable to alter a small amount compared with the large change in area ofthe orifice 58 resulting from movement of half-ball valve 131.Inaccuracy of the mechanical governor 121 with variation in altitude ofthe aircraft can occur as a result of the very substantial change infuel flow with altitude to maintain a particular engine speed. Withincrease in altitude the fuel required to maintain a particular enginespeed will reduce very substantially. The pressure of fuel supplied tothe engine at the pipe 19 will depend on the flow rate of fuel to theengine in accordance with the graph of FIG. 3 and this pressurevariation with variation in fuel flow is reacted on the lever 123through the half-ball valve 131 with the result that with increase inaltitude a desired speed selected on the pilots lever 129 will creepupwardly. This tendency is compensated by a creep control which exerts acompensating force on lever 123. A restrictor potentiometer 130 is fedwith low pressure delivery pressure (P2) from the engine and it deliversinto the pipe 132 a fixed proportion of this pressure. This pressure isfed to a bellows 133 which reacts on an auxiliary lever 134. The bellows133 is opposed by an evacuated bellows 135 to ensure that the forceacting on lever 134 remains in proportion to the absolute value ofpressure in pipe 132. The fulcrum 136 of the lever 134 is adjustable tovary the point at which the lever 134 engages lever 123 relative to itsfulcrum 124. Thus with increase inaltitude the reduced fuel pressureforce actingon the halfball valve 131 is compensated by reduction in thethrust exerted by the bellows 133 on the lever 123, thus compensatingthe governor creep.

An emergency protection must be provided to protect the engine in thecase of excessive turbine temperatures and it is necessary whenever anexcessive temperature occurs for quick action to be taken to reduce fuelflow. In the present fuel system this is conveniently effected throughthe mechanical governor 121. An electric thermocouple temperaturesensing device for the jet pipe of the engine is arranged to operatethrough an amplifier to generate an electric signal dependent on jetpipe temperature which is connected to an electric force motor 137 toexert a corresponding torque on a lever 138. The torque exerted on lever138 is quite small and it is amplified by a hydraulic amplifier devicecomprising opposed jets 139 and 141 and opposed 'mechanically connectedservo-pistons 142 and 143. The lever 138 extends as a flapper inbetweenthe jets 139 and 141 and varies the relative escape flows fromthese jets. The je'ts 139 and 141 receive operating fuel from the servosupply pipe 49 and the relative escape flows at the jets 139 and 141will, by virtue of restrictors, generate pressures which act oppositelyon the pistons 142 and 143. Spring 144'feeds back the movement ofpistons 142 and 143 on to the lever 138 so that the movement of thepistons 142 and 143 is in accordance with the torque exerted by themotor 137. The pistons 142 and 143 react' together through a spring 145on to the control lever 123 in such manner that, on the occurrence ofexcess jet pipe temperature, the spring 145 will exert an extra force onthe lever 123 to lift the halfball valve 131 from the vent 65 to movepiston 53 to reduce the throttle aperture 58 and thus reduce fuel flowto the engine.

Another safety precaution necessary with the engine is the prevention ofexcessive pressure within the combustion chambers. This pressure is thepressure P3 delivered by the high pressure compressor. For this purposea pressure limiter unit 146 is provided. The pressure P3 enters the unitthrough a filter 147 into a chamber 148 where the pressure reacts on abellows 149 connected to receive ambient atmospheric pressure (P This isthe pressure which exists in the engine nacelle. The bellows 149 isconnected to operate on a lever 151 which is pivotally or flexurallymounted at 152. The pressure P3 acting within the chamber 148 escapesfrom the chamber through an orifice 153 which is connected in serieswith a restrictor 1S4 connected back to P pressure. The junction of theorifice 153 and restrictor 154 is connected to pipe 85. The bellows 149is arranged to compress when the pressure P3 reaches an excessive valueto move lever 151 to a position where it interrupts the flow of air intothe orifree 153. Such interruption will reduce thepressure in pipe 85which will reduce the force exerted by bellows 79 in the throttle 52.This reduction in bellows force will upset the balance of the lever 76and cause movement of throttle piston 53 in the sense to reduce thethrottle aperture 58 and thus reduce the fuel flow to the engine in anoverriding manner. 8

The stabilising jets 14, 15, 16, 17 and 18 of the aircraft shown in FIG.1 require the flow of high pressure air from the high pressurecompressor and when the aircraft is in its vertical flight mode, i.e.,the nozzles 6 and 7 are pointed downwardly, the flow of stabilising airwill cause a change in the engine operating conditions which willrequire altered fuel flow to the engine. There are two states of engineoperation to be considered, i.e., engine acceleration operation andengine constant speed operation. During engine acceleration the flow ofair from the high pressure compressor to the stabilising jets will meanthat a smaller quantity of air at a somewhat lower pressure is fed intothe combustion chambers. Clearly if the engine is to maintain the samevertical thrust from the jet nozzles 7 more fuel must be burnt in thereduced quantity of air entering the combustion chambers. To compensatethe fuel system to provide such extra fuel the reset unit 155 isprovided. Basically the unit comprises a throttle piston 156 adjustablymovable in a cylinder through a pair of spaced ports 157 and 158, athrottle aperture 159 within the piston providing a variable connectionbetween the ports depending on the position of the piston. The port 157connects through pipe 161 to the pipe 51 upstream of the throttle 52.The port 158 connects through pipe 162 to the pipe 56 downstream of thethrottle 52. Effectively the unit 155 provides a separate throttle inparallel with the throttle valve 52, which since the pressure dropacross the throttle 52 is controlled by the by-pass valve 91 will ensurean extra fuel flow in accordance with the setting of the piston 156. Thepiston 156 is adjusted by the control signals applied to a pair. ofopposed bellows 163 and 164. Bellows 163 receives pressure P3, from thehigh pressure compressor and the bellows 164 receives the generalpressure PD existing in the ducts 9, 11, 12 and 13. This pressure PDwill vary in accordance with the flow rate of high pressure air to thestabilising nozzles. The two bellows 163 and 164 are connected togetherby a yoke 165 which reacts at a pin 166 on a lever 167 mounted in achamber 168. The lever is pivoted at a fulcrum 169 167 by means of aspring 175 so that movement of piston 156 as a result of alteration ofthe positionof the half-ball valve 171 will restore the, half-ball valve171 into an equilibrium position where the escape flow is such thatthere is a force balance on piston 171 as a result of the differentpressures in chambers 172 and 173.

The selection of air flow in any of the ducts to the stabilising nozzleswill cause a reduction in the pressure PD fed to the bellows 164 whichwill cause lift of the halfball valve 171 to reduce pressure in chamber173. Downward movement'of the piston 156 will increase the size ofaperture 159 to the ports 157 and 158 thus permitting an increase infuel flow into the pipe 56. The shaping of the throttle aperture 159ensures that the increased fuel fed to the engine is in accordance withthe reduction of pressure PD which in turn is in accordance with theactual flow rate of air fed to the stabilising nozzles. By this meansthe acceleration rate of the engine may be kept as high as possibleirrespectively of the amount of air fed to the stabilising nozzles. Theuse of compressor delivery pressure P3 as part of the signal control forextra fuel flow ensures 'a degree of altitude compensation of the extrafuel flow. Also the flyweights 113 will operate to control pressure dropat aperture 159 as at aperture 58 ensuring speed compensation for theextra fuel flow.

The permitted increase in fuel flow to the engine by operation of theunit 155 when air flow is selected for any stabilising jet operatesprincipally to increase fuel flow during the acceleration mode of theengine. To a lesser degree however, the opening of the throttle aperture159 following demand for stabilising jet air flow during constant speedoperation of the engine will also involve a slight resetting of themechanical governor 121. Since the mechanical governor 121 actsoverridingly to control the position of the throttle piston 53 duringconstant speed operation of the engine, the

opening of the extra throttle passage 159 when air flows from thestabilising jets will itself increase fuel flow to the engine which willinitially react on the governor 121 as an increased pressure athalf-ball valve 131. Engine speed will change both as a result ofincreased fuel flow and of the bleed off of high pressure air, and thegovernor 121 will then readjust the throttle valve 52 to give a slightlylower fuel flow rate. The overall effect will be a slight reduction ofthe constant engine speed and a slightly increased total fuel flow tothe engine.

The throttle unit 52 is the first throttle unit which asand carries ahalf-ball valve 160 which controls escape V flow from a vent 170. Aservo-piston 171 secured to the throttle piston 156 moves within acylinder having upper and lower chambers 172 and 173. In the upperchamber 172 servo fuel from pipe 49 acts on a small area of piston 1 71.A restrictor 174 feeds liquid at pressure to the lower chamber 173 wherethe pressure will act on a larger area of piston 171. The vent 170 isalso connected to the chamber 173 and escape flow from the vent 170 willdetermine a lower pressure within the lower chamber 173. Movement of thehalf-ball valve 171 to increase escape flow from the vent 172 willreduce pressure in the lower chamber 173 causing the servo-piston 171 tomove downwardly. The resulting movement of the piston 156 is fed back onto the lever leaving the first throttle unit 52 then passes through thesecond throttle unit 176 before passing to the engine. The secondthrottle unit performs the dual function of a shut-off valve and a fuelscheduling device. The throttle unit 176 comprises a hollow piston 177movable in a cylinder 178 to control the relative flows at four ports179, 181, 182 and 183. The pipe 56 connects to the port 182 and the pipe19 for the engine connects to the port 179. The upper and lower ends ofthe cylinder 178 are connected to the low pressure network 36 so that nohydraulic pressure acts in the endwise sense on the throttle piston 177.The port' 183 also connects to the low pressure network 36. The port 181connects to a pipe 184 to carry fuel b'y-passed from the pipe 56 duringthe controllingfunction exerted by the throttle valve 176.

The throttle piston 177 is movable in the endwise sense by means of alever 185 mechanically connected for movement by the pilots controllever 129. The cam 128 and the lever 185 are moved together by thepilots lever 129. Within the throttle piston 177 there are three ports186, 187 and 188 which connect to the hollow interior of the piston. Theport 186 over the whole range of movement of piston 177 makes anunthrottled connection with port 182. In the shut-off position of thethrottle piston 177 (i.e., its uppermost position) port 186 forms asubstantially unrestricted connection between the ports 182 and 183 toby-pass all delivered fuel back to the low pressure network 36. In thefuel scheduling position of the throttle 177 corresponding to themovement of the pilots lever 129 between idle and full speed (the rangeof movement of the piston 177 except for its uppermost position) theport 188 will make a variable throttled connection with the port 179 andthe port 187 will make a substantially unthrottled connection with theport 181. Thus over the operating range from idle to full speed, fuelflow to the engine passes from pipe 56 through ports 182 and 186 intothe hollow interior of piston 177 and then leaves through two paths, onecomprising the throttle aperture 188 in j the port 179 to feed to theengine, and the other comprisingthe port 187 whereby fuel may passthrough port 181 and 184 to the by-pass valve 189. in the shutoffposition of the valve 176 the port 188 will be closed by the landbetween ports 181 and 179 and the port 187 will connect to the port 182.

A second by-pass valve 189 co-operates with the second throttle valve176. The by-pass valve comprises a flexible diaphragm 191 mounted inbetween upper and.

lower chambers 192 and 193 in the unit 189. The lower chamber 193 isconnected directly to the pipe 184 and the upper chamber 192 isconnected directly to the port 179 via pipe 194. Thus the pressure dropoccurring between port 181 and port 179 due to the engine fuel flowthrough the throttle aperture 188 will be fed to act on opposite sidesof the diaphragm 191 to adjust its position against a spring 195. Thediaphragm 191 controls the by-pass flow of fuel from the chamber 193through a port 196, such by-pass flow from port 196 entering the lowpressure network 36. The action of the diaphragm 191 therefore is tocontrol pressure drop occurring between ports 181 and 179 in thethrottle unit 176 to a substantially constant value which is inaccordance with the loading of the spring 195. Under some circumstancesof operation the fuel flow through the throttle unit 176 will not besufficient to produce a pressure drop which will open the by-pass valve189, in which case all the fuel entering the second throttle unit 176will leave through the pipe 19 to the engine.

For the purpose of providing a minimum idling flow of fuel to theengine, the by-pass valve 189 is provided with an adjustable restrictor197 to carry fuel between the pipes 194 and 184 parallel with the flowpath presented by the throttle aperture 188. The restrictor 197 thenprovides a minimum value for the throttle aperture 188 to maintain aminimum fuel flow to the engine under idling conditions. In the shut-offposition for the throttle valve 176 the port 181 will be isolated sincethe port 187 is then in register with port 182 and therefore all flow tothe engine is cut off. Also the by-pass port 181 is closed.

Conventional means for water injection may be provided in the engine onoperation of a suitable control by the pilot in order to lower turbinetemperatures for short periods during high power operation. The meansfor water injection concern the present invention in so far that thefuel system must be capable of adjustment whilst water is injected intothe engine to supply a greater controlled fuel flow. The water injectionsystem for the engine includes a signalling switch operated as a resultof water injection to the engine, such switch being arranged to energisea pair of solenoids 198 and 199 in the fuel system. The solenoid 198when energised will open a valve 201 between two pipes 202 and 203. Thepipe 202 connects to the pipe 19 delivering fuel to the engine whilstthe pipe 203 extends to an upper auxiliary valve section 204 of throttleunit 176.

Valve section 204 is really a shut-off valve which opens connectionbetween the pipe 203 and a pipe 205 over the whole range of movement ofthe pilot's throttle 129 with the exception that the connection is cutoff when the control handle reaches the shut-off position. The pipe 205isconnected to the servo supply pipes-49 which in turn connects back tothe filter 48 at the junction of pipes 38 and 51. Therefore when thesolenoid 198 is energised to open valve 201a. connection is opened fromthe pipe 51 to the pipe 19 providing a bypass circuit for the twothrottle valves 52 and 176. The flow rate of fuel in this by-passcircuit is determined firstly by the total pressure drop controlledacross the two throttle valves 52 and 176 and the restrictive effect ofthe valve 201. The valve 201 is particularly concerned with operationalconditions during engine acceleration with water injection. The valve199 is also energised during water injection and operates to load aspring 206 on to the lever 123 of the mechanical governor in the senseto raise the governed speed of the engine and thereby cause more fuel toflow to the engine. Water injection is used solely duringlow levelflying, principally during take-off, to enable the maximum amount ofengine power to be obtained. The water injection permits more fuel to beinjected into the engine, the cooling effect of the water thenpreventing excessive rise of turbine temperature.

For normal engine accelerations the pilots lever 129 I is moved toselect an increased speed, such movement jointly increasing the load onthe governor spring 127 and increasing the effective size of throttleaperture 188. Increasing the load of the governor spring will closehalf-ball valve 131 on to the vent 65. The servopiston 59 controllingthe throttle piston 53 is then under the control of the half-ball valve78 reacting through the diaphragm 71 of the discriminator 67. Theservo-piston 59 will then move piston 53 to a position determined by theengine gas pressures reacting on the bellows 79 and 81 and the by-passvalve 91 will operate to control by-pass in accordance with the squareof speed of the high pressure spool. Thus the fuel flow through thethrottle 52 is sufficient for safe engine acceleration. The engine willthen accelerate until the fuel flow is limited either by the throttlevalve 176 or by the mechanical governor. If the selected position of thepilots lever 129 is less than about percent of the full speed position,it is more likely that the flow limit of the throttle 176 will beattained before the mechani-,

cal governor reaches a controlling speed. The fuel flow to the engine isthen limited when the pressure drop of fuel flow through the throttleaperture 188 reaches a value which overcomes the pressure loading ofspring on diaphragm 191 at which point the diaphragm will lift from thevent 196 to allow by-pass from the fuel delivered to valve 176 so thatthe flow leaving valve 176 through pipe 19 is sufficient only to producethe fixed pressure drop determined by the by-pass valve 189. By-passedfuel goes back to the low pressure network 36. If however, the pilotslever 129 is set at a higher position for example above 80 percent ofthe maximum position it is more likely that the engine speed will belimited by the mechanical governor in that the centrifugal forcegenerated by flyweights 124 on rod 126 will begin to overcome theloading of the spring 127 thus lifting the half-ball valve 131 from vent65. When this happens the slightly lowered pressure in the lower chamber69 of throttle valve 52 will cause the flexible diaphragm 71 to closethe vent 72 so that the reduced pressure determined in the vent pipe 65will act solely in the lower end 64 of servo cylinder 61 to control theposition of the throttle piston 53. Since the pressure drop across thethrottle valve 52 is capable only of slow variation, the action of theflyweights acting against the spring 127 will control engine speedaccurately by determining the size of the throttling aperture 58 inwhich fuel flows to the engine.

The arrangement of the two throttle valves 52 and 176 is such that forall engine speeds above about 85 percent of maximum speed the mechanicalgovernor must take control, the fuel flow permitted by the mechanicalgovernor when it passes through the throttling aperture 188 of throttlevalve 176 not producing a sufficient pressure drop to open the by-passvalve 189.

For selected lever positions below 85 percent the ultimate control ofthe engine will be either by the throttle valve 176 or by the mechanicalgovernor, depending on the altitude of operation and the resulting fuelflow necessary to maintain a selected speed. For quite low selectedspeeds the throttle valve 176 will normally effect control since the cam128 is so arranged that for low percentage positions of the controllever 129 the effective speeds selected by the mechanical governor isalways such as to demand a greater fuel flow to the engine than would bepermitted by the setting of the throttle valve 176 for that position oflever 129. There is however, a range of positions for the control lever129 lyingbetween about 60 and 85 percentof the full speed position inwhich either the throttle 176 or the' mechanical governor 121 willcontrol engine speed in the constant speed state, this depending on thealtitude of flight. With increase in altitude the fuel flow necessary tomaintain a particular constant speed of the engine will become lower andtherefore with increase in altitude the range of speeds over which themechanical governor will control will extend more and more from the 85percent lever position back towards the 60 percent lever position. ltwill however be seen that engine speed is smoothly adjustable bymovement of the control lever 129 quite irrespectively of the altitudeof flight. More particularly, however, the upper speed range above about85 percent is always under accurate governor control. Since the creep ofthe governor is compensated by the creep control device 131, 133, 135,the full speed position of the lever 129 controls the maximum safe speedfor the engine under normal operating conditions. Under water injectionoperation it is true that the mechanical governor would be set to ahigher speed but the higher speed is then permissible because of thetemperature reducing effect of the water on the turbine.

The flyweights 124 in the mechanical governor may be driven by the highor the low pressure spool in the two-spool engine but as describedflyweights 124 are driven by the low pressure spool.

An emergency system is provided for the fuel system which comprises thechange-over valve 37, the emergency throttle unit 206 and the emergencyby-pass valve 207. The emergency throttle valve 206 comprises a hollowthrottle piston 208 arranged to control flow between a pair of ports 209and 211. The hollow piston 208 includes a port 212 constantly inconnection with the port 21 1 during piston movement and a throttleaperture 213 in overlapping relation with the port 209 so that itseffective throttle aperture is varied during piston movement. The pistonis adjusted in the axial direction by means of a lever 214 which ismechanically connected for movement with the pilots lever 129.

The by-pass valve 207 includes an upper and a lower chamber 216 and 217separated by a flexible diaphragm 218 loaded by spring 219 on to a vent221 connected to the low pressure network 36.

The port 209 in the emergency throttle 206 connects to a pipe 222 whichextends to connect into the engine pipe 19. A non-return valve 223 islocated in the pipe 19 between the throttle valve 176 and the junctionof vpipe 19 with pipe 222.

if under any circumstances of operation it appearsto the pilot that thefuel system is not operating correctly, the pilot will press anemergency button to energise the emergency solenoid to open valve 44 andcause the change-over valve 37 to connect pump delivery 33 to theemergency pipe 39. From the emergency pipe'39 fuel enters the port 211of the emergency throttle and enters the hollow piston 208 through port212. Fuel leaves through the throttle aperture 213 into a port 209 andpipe 222.'The pressure drop occurring at the throttle aperture isconnected to pipe 39 to the lower chamber 217 and through pipe 224 tothe upper chamber 216 of the emergency by-pass 207 and if the pressuredrop is sufficient to overcome the loading of the spring 219 theflexible diaphragm 218 will move to open the seat 221 so that fueldelivered from the emergency pipe 39 may passinto the low pressurenetwork 36, the remaining fuel leaving the emergency throttle being justsufficient to maintain a pressure drop at the throttle aperture 213 tohold the diaphragm 218 in its by-passing condition. The emergency systemcomprises principally the simple flow control formed by the emergencythrottle and the emergency by-pass and the fuel flow permitted to theengine by this emergency system is arranged to be about the same as thefuel flow permitted to the engine by the second throttle 176. Onswitching over the emergency system the pilot will lose the variousrefined controls for his engine and clearly must adjust the emergencythrottle very carefully in order to adjust engine speed. Nevertheless,the emergency throttle 208 and emergency by-pass 207 represent a simplebut effective fuel control system. The nonreturn valve 223 is essentialto prevent metered fuel from the emergency system from leaking back intothe normal system.

I claim:

1. A fuel system for a gas turbine engine including a fuel flow sourceconnected to deliver fuel to the engine, a first smoothly adjustablefuel flow means to vary fuel flow to the engine, a second smoothlyadjustable fuel flow means to vary fuel flow to the engine, a singlemanually operable means acting jointly on the first and second fuel flowmeans, a third smoothly adjustable fuel flow means responsive to theinstantaneous value of at least one engine function to determine fuelflow to the engine during engine acceleration following movement of saidmanually operable means to increase the fuel flow to the engine, andmeans ensuring that the fuel flow fed to the engine is the lowest of theflows selected by the three fuel flow means, said first fuel flow meansincluding a flyweight means driven at a speed proportional to enginespeed, and adjustable spring means acting on the flyweight device tooppose centrifugal movement at least over the upper range of enginespeeds to adjust the fuel flow permitted by the first fuel flow means,said manually operable means acting to adjust the spring means.

2. A fuel system for a gas turbine engine as claimed in claim 1 whereinthe second smoothly adjustable fuel flow means includes a secondvariable throttle through which fuel passes to the engine and a secondby-pass valve arranged to by-pass flow from the throttle to maintain acontrolled pressure drop at the throttle and wherein the first and thethird variable flow means include a common variable throttle and acommon bypass valve together with discriminator means to select one ofsaid flyweight device and said engine function responsive means toadjust said common variable throttle.

3. A fuel system for a gas turbine engine as claimed in claim 2 whereinthe common by-pass valve includes means responsive to engine speedarranged to adjust by-pass fueld flow from the common variable throttleso that the pressure drop at the common variable throttle is inaccordance with the square of engine speed.

4. A fuel system for a gas turbine engine as claimed in claim 3including spring means associated with said common by-pass valve todetermine a minimum fixed pressure drop across the said common variablethrottle in order to determine a minimum fuel flow to the engine.

5. A fuel system for a gas turbine engine as claimed in claim 4 whereinsaid means responsive to engine speed comprises a chamber, meanscarrying fuel to said chamber, flyweights within the chamber, a driveconnection from the engine to the flyweights, and connecting meansarranged to transmit centrifugal movement of the fiyweights to adjustsaid common by-pass valve whereby by-pass flow maintains a pressure dropat the common throttle directly in accordance with engine speed squaredand inversely in accordance with fuel density.

6. A fuel system for a gas turbine engine as claimed in claim 5including a piston device adjusting the common by-pass valve andsubjected to the pressure drop of the common variable throttle inopposition to. the centrifugal force of the flyweights.

7. A fuel system for a gas turbine engine as claimed in claim 1including means relating relative adjustment of the spring means and thesecond smoothly adjustable fuel flow means comprising a cam connectingthe manually operable means to adjust the spring means and a directmechanical connection connecting the manually operable means to thesecond adjustable fuel flow means. 7

8. A fuel system for a gas turbine engine as claimed in claim 2 whereinthe variable by-pass valve forming part of the second adjustable fuelflow means is arranged to by-pass fuel from the variable throttleassociated with the second adjustable fuel flow means to maintain asubstantially constant pressure drop across the throttle, said throttleassociated with the second variable by-pass valve also including a fuelshut-off position to shut off fuel flow when the said manually operablemeans is moved to a fuel shut off position.

9. A fuel system for a gas turbine engine as claimed in claim 1including an emergency change over valve connected to receive fuel fromsaid source and alternatively operable to feed fuel through the first,second and third smoothly adjustable fuel flow means to the engine or tofeed fuel through an emergency smoothly adjustable fuel flow means tothe engine, and nonreturn valve means connected to prevent fueldelivered by the emergency fuel flow means from entering the first,second and third fuel flow means.

10. A fuel system for a gas turbine engine as claimed .in claim 9wherein the manually operable means is also arranged to adjust theemergency fuel flow means simultaneously with the first and second fuelflow means.

1 l. A fuel system for a gas turbine engine as claimed in claim 10wherein the emergency fuel flow means comprises an emergency variablethrottle adjusted by the manually operable means and an emergency bypassvalve arranged to by-pass fuel from the flow entering the emergencytrotttle valve to maintain a substantially constant pressure drop acrossthe emergency throttle valve, the emergency throttle valve alsoincluding a shut off position to shut off fuel flow to the engine whenthe said manually operable means occupies its shut off position.

1. A fuel system for a gas turbine engine including a fuel flow sourceconnected to deliver fuel to the engine, a first smoothly adjustablefuel flow means to vary fuel flow to the engine, a second smoothlyadjustable fuel flow means to vary fuel flow to the engine, a singlemanually operable means acting jointly on the first and second fuel flowmeans, a third smoothly adjustable fuel flow means responsive to theinstantaneous value of at least one engine function to determine fuelflow to the engine during engine acceleration following movement of saidmanually operable means to increase the fuel flow to the engine, andmeans ensuring that the fuel flow fed to the engine is the lowest of theflows selected by the three fuel flow means, said first fuel flow meansincluding a flyweight means driven at a speed proportional to enginespeed, and adjustable spring means acting on the flyweight device tooppose centrifugal movement at least over the upper range of enginespeeds to adjust the fuel flow permitted by the first fuel flow means,said manually operable means acting to adjust the spring means.
 2. Afuel system for a gas turbine engine as claimed in claim 1 wherein thesecond smoothly adjustable fuel flow means includes a second varIablethrottle through which fuel passes to the engine and a second by-passvalve arranged to by-pass flow from the throttle to maintain acontrolled pressure drop at the throttle and wherein the first and thethird variable flow means include a common variable throttle and acommon by-pass valve together with discriminator means to select one ofsaid flyweight device and said engine function responsive means toadjust said common variable throttle.
 3. A fuel system for a gas turbineengine as claimed in claim 2 wherein the common by-pass valve includesmeans responsive to engine speed arranged to adjust by-pass fueld flowfrom the common variable throttle so that the pressure drop at thecommon variable throttle is in accordance with the square of enginespeed.
 4. A fuel system for a gas turbine engine as claimed in claim 3including spring means associated with said common by-pass valve todetermine a minimum fixed pressure drop across the said common variablethrottle in order to determine a minimum fuel flow to the engine.
 5. Afuel system for a gas turbine engine as claimed in claim 4 wherein saidmeans responsive to engine speed comprises a chamber, means carryingfuel to said chamber, flyweights within the chamber, a drive connectionfrom the engine to the flyweights, and connecting means arranged totransmit centrifugal movement of the flyweights to adjust said commonby-pass valve whereby by-pass flow maintains a pressure drop at thecommon throttle directly in accordance with engine speed squared andinversely in accordance with fuel density.
 6. A fuel system for a gasturbine engine as claimed in claim 5 including a piston device adjustingthe common by-pass valve and subjected to the pressure drop of thecommon variable throttle in opposition to the centrifugal force of theflyweights.
 7. A fuel system for a gas turbine engine as claimed inclaim 1 including means relating relative adjustment of the spring meansand the second smoothly adjustable fuel flow means comprising a camconnecting the manually operable means to adjust the spring means and adirect mechanical connection connecting the manually operable means tothe second adjustable fuel flow means.
 8. A fuel system for a gasturbine engine as claimed in claim 2 wherein the variable by-pass valveforming part of the second adjustable fuel flow means is arranged toby-pass fuel from the variable throttle associated with the secondadjustable fuel flow means to maintain a substantially constant pressuredrop across the throttle, said throttle associated with the secondvariable by-pass valve also including a fuel shut-off position to shutoff fuel flow when the said manually operable means is moved to a fuelshut off position.
 9. A fuel system for a gas turbine engine as claimedin claim 1 including an emergency change over valve connected to receivefuel from said source and alternatively operable to feed fuel throughthe first, second and third smoothly adjustable fuel flow means to theengine or to feed fuel through an emergency smoothly adjustable fuelflow means to the engine, and non-return valve means connected toprevent fuel delivered by the emergency fuel flow means from enteringthe first, second and third fuel flow means.
 10. A fuel system for a gasturbine engine as claimed in claim 9 wherein the manually operable meansis also arranged to adjust the emergency fuel flow means simultaneouslywith the first and second fuel flow means.
 11. A fuel system for a gasturbine engine as claimed in claim 10 wherein the emergency fuel flowmeans comprises an emergency variable throttle adjusted by the manuallyoperable means and an emergency by-pass valve arranged to by-pass fuelfrom the flow entering the emergency trotttle valve to maintain asubstantially constant pressure drop across the emergency throttlevalve, the emergency throttle valve also including a shut off positionto shut off fuel flow to the engine when the said manually operablemeans occupies its shut off position. >